The present invention relates to a pressure control apparatus for an ink-jet pen. The apparatus will adjust the back pressure of ink in the reservoir of the inkjet pen within a suitable range when the ink is used gradually.
Drop-on-demand printing is a general method for controlling ink drops from an inkjet pen reservoir to a printing surface. The print heads typically use thermal bubble or piezoelectric pressure wave mechanisms for ejecting drops. A thermal bubble type print head includes a thin-film resistor that is heated to cause sudden vaporization of a small portion of the ink. The rapid expansion of the ink vapor forces a small amount of ink through a print head orifice. Though a drop-on-demand print head is effective for ejecting ink drops from a pen reservoir, it needs another control mechanism for preventing ink from permeating through the print head when the print head is inactive. Generally the control mechanism is to provide a slight back pressure at the print head to prevent ink leakage from the pen whenever the print head is inactive. The term "back pressure" means the partial vacuum within the pen reservoir that resists the flow of ink through the print head. Back pressure is considered in the positive sense so that an increase in back pressure represents an increase in the partial vacuum.
The back pressure at the print head must be strong enough for preventing ink leakage. However, it must not be so strong that the print head is unable to overcome the back pressure to eject ink drops. Moreover, the back pressure must be adjusted itself according to the variance of environmental air pressure. For example, during air transport of an inkjet pen, the ambient air pressure decreases as the aircraft gains altitude, a correspondingly greater amount of back pressure is needed to keep ink from leaking through the print head. On the other hand, the back pressure within an ink-jet pen reservoir is also affected by "operational effects". As the print head is activated to eject ink drops, the consequent depletion of ink from the reservoir increases the reservoir back pressure. If the back pressure increase is not well regulated, the ink drops will reduce their size, lose printing quality or even fail to be ejected because the print head is unable to overcome the increased back pressure.
In the prior arts, a back pressure control mechanism is an accumulator mounted inside the ink-jet pen reservoir. The accumulator is usually an expandable bag capable of changing its volume between a minimum volume and a maximum volume, therefore to adjust the reservoir volume for storage of ink, and to regulate the back pressure of the reservoir. For example, when the ambient air pressure decreases, the expandable bag will then be contracted to reduce its volume and relatively to increase the reservoir volume for storage of ink, and therefore to increase the back pressure of reservoir for preventing ink from leakage.
A prior art is disclosed in U.S. Pat. No. 5,409,134. As shown in FIG. 1 and FIG. 2, an accumulator 14 is configured to fit into an ink-jet pen 10 that includes a cap 12, a reservoir 11 having rigid side walls 111 and a bottom wall 112 that are configured to hold a quantity of ink. A thermal-bubble type print head 13 is fit into the bottom wall 112 of the reservoir 11 for ejecting ink drops from the reservoir 11 to an unshown print paper. The accumulator 14 includes an expandable bag 142a, 142b that is mounted to a spring 141a, 141b. The bag 142a, 142b and spring 141a, 141b are aligned with pins 147 and fastened via a base 145 to the cap 12 in a manner that the interior of the bag 142a, 142b is in fluid communication with ambient air through a breather strip 148 and an air duct 149. With the accumulator 14 in place, the reservoir 11 is filled with ink 50 with a slight (minimum) back pressure established within the pen reservoir 11. The minimum back pressure is the back pressure necessary to keep ink 50 from leaking through the print head 13 when the print head is inactive.
As the pen 10 is used for printing, the air pressure within the reservoir 11 decreases (hence, the back pressure increases) as ink is depleted. As shown in FIG. 1, during printing, the bag 142a, 142b expands as a result of the back pressure increase. The bag expansion decreases the volume of the reservoir 11 to maintain the reservoir back pressure within a range such that the print head 13 is able to continue ejecting ink from the reservoir 11. As shown in FIG. 2, if the ambient pressure should thereafter decrease (for example, during air transport of the pen), the bag 142a, 142b will contract to increase the reservoir volume so that the back pressure within the reservoir 11, relative to ambient, does not drop to a level that permits ink 50 to leak from the print head 13.
From the aforesaid function of the accumulator 14, we may find that the maximum volume of the bag 142a, 142b is limited. That is, when the bag 142a, 142b has expanded to its maximum volume, the further depletion of the ink 50 will increase the back pressure of the reservoir 11 to an extent that the print head 13 cannot overcome the pressure to inject ink drops and the ink 50 cannot be fully depleted.
To solve the aforesaid problem, U.S. Pat. No. 5,526,030 discloses a bubble generator as a back pressure control mechanism. As shown in FIG. 3 and FIG. 4, a bubble generator 20 is installed in the bottom 112a of a reservoir 11a of an ink-jet pen 10. The bubble generator 20 consists of a tubular boss 21 and a sphere 22 mounted concentrically within the boss. The outside diameter of the sphere 22 is smaller than the inside diameter of the boss 21 to define an annular orifice 212. The sphere 22 is maintained within the boss 21 by a number of raised ribs 211 formed around the interior of the boss 21. In this manner the sphere 2 can be easily press fit into the boss 21 and firmly maintained in position by the ribs 211. Additional unshown raised ribs are also provided to help maintaining the sphere 22 in position away from the inside wall of the boss 21. The sphere 22 serves as a capillary member to maintain a quantity of ink within the boss 21. As a result, even when the pen is oriented such that the boss is not submerged in the reservoir ink, a quantity of ink is trapped within the boss to provide a liquid seal. When the back pressure is within the desired range, the bubble generator 21 is sealed with a quantity of ink. However, when the back pressure exceeds the desired range, the back pressure overcomes the capillary forces of the liquid seal and allows the ambient air to bubble into the reservoir 11a to reduce the back pressure. When the back pressure returns to the appropriate level, the liquid seal reforms to prevent further ingression of the ambient air.
Therefore, the bubble generator 20 applied in the ink-jet pen 10 must have the capabilities of:
a) A suitable control of bubble ingression for maintaining a suitable back pressure of the reservoir. That is, after bubbles ingressing into the reservoir 11a and decreasing the back pressure, the bubble generator 20 has to stop the ingression at a right time in order to maintain a suitable back pressure of the reservoir 11a; and
b) A self-wetting function for the mechanism that when the ink 50 in the reservoir 11a has been depleted to an extent or the bubble generator 20 is not submerged in the reservoir ink due to any angular placement of the ink-jet pen 10, the liquid seal should always functions to prevent bubble ingression and maintain a suitable back pressure of the reservoir 11a.
As described above, since the volume of an expendable bag is limited. When the bag has expanded to its maximum volume, the further depletion of the ink will increase the back pressure of the reservoir to an extent that the print head cannot overcome the back pressure to eject ink drops and the ink cannot be fully depleted.
On the other hand, a bubble generator for controlling back pressure of the ink reservoir is based on the capillary forces of the liquid seal, the static pressure of the ink and the configuration of the orifice. A greater capillary force or a smaller orifice clearance requires a higher back pressure for introducing bubble. When the ink is decreased, the static pressure of the ink and the back pressure for introducing bubble are also decreased. Therefore, the demand of maintaining a suitable back pressure of ink reservoir during bubble ingression requires a precise control of the configuration of orifice, i.e. the clearance between the sphere and the boss, which causes difficulties in manufacturing and assembly.
The major objective of the present invention is therefore to provide a back pressure control mechanism for an ink-jet pen, and to solve the aforesaid problems of prior arts. The mechanism will maintain the back pressure in a preset range and prevent it from too high and influencing printing quality of the print head.
A back pressure control mechanism according to the present invention includes a spring element, a pressure adjusting element and a hole furnished on the bottom wall of a reservoir. The pressure adjusting element is installed inside the hole to form a seal for bubble ingression. The spring element connects to the pressure adjusting element for providing contact pressure of the pressure adjusting element to the hole. Under normal condition, the pressure adjusting element contacts tightly to the hole and prevents air from ingression. When the back pressure increases gradually to an extent, the ambient air will overcome the pressure of a spring which provides force to the pressure adjusting element, thus move the pressure adjusting element to leave a clearance to the hole and let some air bubbles ingressing into the reservoir till the back pressure decreases to a certain value. After the back pressure returns to the certain value, the pressure adjusting element further contacts the hole and prevents air from more ingression.
The features and advantages of the present invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings.